Your system is, indeed, an old classic – with its usual problems. Your present problem began with the leak at the pipe and ends with its repair. There is a long-winded explanation why this occurs. In short, the leak hole made another expansion tank so air could be drawn into the system as water was forced into the existing expansion tank. A more complete explanation can be read at http://www.hydronic.net
at the Lessons section under ‘Make a Positive Pressure’
The extra volume of air causes the relief valve weeping as the system water heats up. Your Dad’s system worked for many years while there was no leak, so the bladder expansion tank change is not necessary. There should be a gauge on the boiler (furnace means warm air appliance) so the old gauge should be replaced with the new at that location.
The existing expansion tank is also called an air control tank. It is placed at that location to trap any air in the system and act as an air scoop to catch the air bubbles within the sealed system.
Changing the old tank to the bladder tank changes the system to an air elimination system. The bladder tank is not meant to trap air in the waterside of the system. The air has to be eliminated or vented at a convenient spot, usually at the boiler or an air scoop right off the supply side of the boiler. You can install the bladder expansion tank at any convenient distance, it doesn’t move much water. It is a good idea to place the bladder tank and its piping below the main with no air traps in the line, so air bubbles go back to the system air vent. nIstall the bladder tank with the fitting upwards so bubbles go up the line to the main.